This invention relates to a self-adaptive echo canceller for use in self-adaptively cancelling an echo in a long-distance telephone network, more particularly, in a satellite communication network.
In a long-distance telephone network, a long delay inevitably takes place on communication and gives rise to an echo in a speech channel. Such an echo should be suppressed because it deteriorates quality of communication. For this purpose, use was made of an echo suppressor which is for suppressing the echo by switching a speech channel on and off. The echo suppressor, however, causes troubles, such as occurrence of click noises, an initial interruption of a speech, or the like.
A conventional self-adaptive echo canceller has been proposed so as to avoid the above-mentioned troubles. An echo canceller for use in connection to a subscriber's substation has a receive-in terminal for receiving a receive-in signal from a remote party, a receive-out terminal for supplying the substation with a receive-out signal, a send-in terminal for receiving a send-in signal from the substation, and a send-out terminal for sending a send-out signal to the remote party. In the manner known in the art, the echo canceller is for cancelling an echo signal which results from the receive-in signal between the receive-out and the send-in terminals. In this matter, and echo path is formed between the receive-out and the send-in terminals. As a result, the send-in terminal is supplied with the echo signal resulting from the receive-in signal.
As will later be described in conjunction with one figure of the accompanying drawing, the conventional self-adaptive echo canceller provides a model of the echo path by the use of a learning identification technique to produce an echo estimation signal approximate to the echo signal and cancels the echo signal by subtracting the echo estimation signal from the echo signal.
It should be noted here that a degree of cancellation of the echo signal is reduced with the self-adaptive echo canceller as a level of the receive-in signal is lowered. In addition, noise occurs in the echo path and in the echo-canceller itself. As a result, the influence of the noise becomes serious when the receive-in signal is weak. In other words, the degree of cancellation is degraded as the level of the receive-in signal becomes low.